In many conventional twin drive rolling mills in which a top reduction roll and a bottom reduction roll are driven respectively by an upper electric motor and a lower electric motor, there has been added load balance control which involves performing the proportional-integral control (PI control) of a difference in the upper and lower load current values supplied to the electric motors or a difference between the load current square values and performing feedback to a front portion of a speed control loop or a front portion of a current control loop (refer to Patent Document 1, for example) in order to make uniform the loads of the upper and lower electric motors that have each an independent control system.
However, when an imbalance amount of upper and lower loads increases due to the material entry condition, rolled material shape and the like during rolling, it is impossible to control the above-described differences by the conventional load balance control and there are cases where the torque current reference of the electric motor that drives the roll on the high load side reaches its limit value, whereas a large margin remains in the torque current reference of the electric motor that drives the roll on the light load side, thereby posing the problem that rolling torque becomes insufficient and a speed decrease amount during material entry increases. In particular, when differential peripheral speed control that involves positively giving a speed difference to the top and bottom rolls during material entry is performed in order to prevent a rolled material whose leading end is bowed upward from striking against table rollers, this phenomenon occurs remarkably due to a high load during material entry and an increase in the imbalance amount.
Patent Document 1: Japanese Patent Laid-Open No. 9-295016